The present invention generally relates to methods and circuits for controlling head position, and more particularly to a method of controlling head position in a magnetic disk unit in which a voice coil motor is used to drive a head during a seek operation, and a circuit for controlling the head position in the magnetic disk unit.
When controlling the head position during a seek operation in which a voice coil motor is used to drive the head, it is desirable to move head at a high velocity and to minimize the seek error rate.
FIG. 1 shows an example of a head positioning circuit of a conventional magnetic disk unit. In FIG. 1, a voice coil motor 43 drives a servo head (magnetic head) 42 which reads servo information from a magnetic disk 41, and the head position is determined by the voice coil motor 43. A position signal generating circuit 44 generates a position signal Ps from a signal which is read by the servo head 42, and supplies the position signal Ps to a velocity detection circuit 45, a position error detection circuit 46 and a track crossing pulse generating circuit 47. The magnetic disk 41 is one of a plurality of magnetic disks which are arranged coaxially.
The velocity detection circuit 45 detects an actual velocity Vr from the position signal Ps and a detection circuit Ic which will be described later, and supplies the actual velocity Vr to a velocity error detection circuit 48. The velocity error detection circuit 48 detects a velocity error signal .DELTA.V from the actual velocity Vr and a target velocity Vc, and supplies the velocity error signal .DELTA.V to a power amplifier and switching part 49. A control current Is which is output from the power amplifier and switching part 49 drives the voice coil motor 43 and is detected by a control current detection circuit 50 which generates the detection current Ic described above. This detection current Ic is supplied to the velocity detection circuit 45 as described above and is also supplied to the position error detection circuit 46. The position error detection circuit 46 detects a position error signal .DELTA.P from the position signal Ps and the detection current Ic, and supplies the position error signal .DELTA.P to the power amplifier and switching part 49.
The track crossing pulse generating circuit 47 generates a track crossing pulse from the position signal Ps, and supplies the track crossing pulse to a main control part 51 which is formed from a micro processor unit (MPU). The main control part 51 generates the target velocity Vc depending on a moving quantity of the servo head 42, monitors the position of the servo head 42 from the track crossing pulse, and supplies to the power amplifier and switching part 49 a coarse/fine switching signal which switches the control from a coarse velocity control to a fine position control in a vicinity of the target position. The switching part of the power amplifier and switching part 46 switches the velocity error signal .DELTA.V from the velocity error detection circuit 48 or the position error signal .DELTA.P from the position error detection circuit 46 in response to the coarse/fine switching signal, and outputs the control current Is.
The velocity detection circuit 45 and the velocity error detection circuit 48 form a velocity control part. On the other hand, the position error detection circuit 46, the track crossing pulse generating circuit 47, the power amplifier and switching part 49, the control current detection circuit 50 and the main control part 51 form a position control circuit.
When the main control part 51 receives the moving quantity (number of tracks) of the servo head 42, the main control part 51 generates the target velocity Vc which is dependent on the moving quantity and supplies the target velocity Vc to the velocity error detection circuit 48. As a result, the control current Is is supplied to the voice coil motor 43 via the velocity error detection circuit 48 and the power amplifier and switching part 49, so as to drive the servo head 42. When it is detected via the position signal generating circuit 44 and the track crossing pulse generating circuit 47 that the servo head 42 has reached a position in the vicinity of the target position, the power amplifier and switching part 49 is switched to the position control side. As a result, the position control of the servo head 42 is carried out depending on the position error signal .DELTA.P which is output from the position error detection circuit 46, and the servo head 42 is positioned to the target track.
The above described head positioning control is carried out by the procedure shown in FIG. 2. When the seek operation is started in FIG. 2, a step S1 sets the moving quantity of the servo head 42. A step S2 starts the velocity control of the servo head 42, and a step S3 decides whether or not the velocity control is ended. When the decision result in the step S3 becomes YES, a step S4 starts the position control of the servo head 42. A step S5 decides whether or not the position control is ended, and the process ends when the decision result in the step S5 becomes YES.
However, when the position control of the servo head 42 is carried out in the magnetic disk unit, the resonance of the servo head 42 may occur. The conventional positioning control circuit delays the access time with respect to a seek instruction which is received within a predetermined time after the resonance of the servo head 42 is detected, and suppresses the resonance by providing a constant resonance attenuation time. For this reason, there is a problem in that the access time is inevitably delayed every time the seek instruction is received within the predetermined time after the resonance of the servo head 42 is detected.
FIG. 3 shows an example of the scanning locus SC of the servo head 42 when the seek operation is carried out. In FIG. 3, CFS denotes a switching time when the coarse/fine switching signal from the main control part 51 undergoes a transition, t1 denotes a time when the positional error of the servo head 42 falls within a certain range (for example, .+-.3 .mu.m), and t2 denotes a time when the seek operation is ended. For example, the time t2 occurs 1.0 ms after the time t1. As may be seen from FIG. 3, even if a seek instruction is received between the times t1 and t2, the access time is delayed to a time after the time t2 so that the resonance of the servo head 42 is suppressed.
However, although the resonance is suppressed to a certain degree by the provision of the settling time of 1.0 ms, the access time becomes long. Particularly when two seek operations are made immediately after one another, the resonance of the servo head 42 is accumulated, and it takes considerable time for the positional error of the servo head 42 to fall within the certain range.